Conventionally, as illustrated in FIG. 7, a device including a first housing 102 for defining a bypass flow passage 101 that takes in a part of air which flows inside a duct, a second housing 103 for attaching the first housing 102 to the duct, and the flow sensor 104 that measures a flow rate of air which flows through the bypass flow passage 101, is known for an air flow measuring device 100 (see JP-A-2010-261771).
When the first housing 102 is obtained as a primarily shaped product, and the second housing 103 is secondarily formed, and then the first housing 102 and the second housing 103 are integrated, because the first housing 102 includes a hollow part which constitutes the bypass flow passage 101, there is concern that the first housing 102 may be deformed or damaged due to the pressure of secondary formation resin injected at the time of the secondary formation. If the first housing 102 is deformed, the shape of the bypass flow passage 101 changes. Accordingly, the passage 101 does not have the shape of the bypass flow passage 101 as designed, so that desired output characteristics of the flow sensor 104 cannot be obtained.